1. Field of the Invention
The present invention relates to an optical absorption analyzer for determining the concentrations of components to be determined by means of an optical absorption method; in particular the present invention relates to an optical absorption analyzer which is capable of determining the components such as ozone and the like which have extremely low concentrations and which are contained within the atmosphere.
2. Description of the Prior Art
In the determination of the concentration of ozone contained within the atmosphere by means of an optical absorption method, ozone, having a concentration of 1 ppm, for example, only absorbs about 0.1% of the quantity of transmitted light. However, the strength of the light source used also fluctuates by 0.1% which is the same order of magnitude as the absorption of light by the ozone. Accordingly, it is impossible to determine the concentration of ozone by means of the usual optical absorption method.
Japanese unexamined patent application (laid-open No.: 29,176/1976) discloses a measuring circuit for determining the concentration of ozone so as to eliminate the effects of fluctuations in the strength of the light source. In principle, this circuit, as shown in FIG. 1, consists of a detector 2 for detecting light transmitted through a measuring cell 1, a detector 4 for directly detecting light emitted from a light source 3, voltage to frequency converters 5 and 6 for converting the voltage signals generated by the detectors 2 and 4, and up/down counters 7 and 8 for counting the pulse signals output from the converters 5 and 6. In operating this circuit, a zero gas is fed into the measuring cell 1 prior to the measurement determination. The signals, which are obtained from detectors 2 and 4 and then converted to pulse signals by the voltage/frequency converters 5 and 6, are then counted up by counters 7 and 8. Counters 7 and 8 are stopped when the count stored in the counter 7, (as shown by a polygonal line 7' in FIG. 2(a); the polygonal line 8' shown in FIG. 2(b) showing the count stored in the counter 8), reaches a definite value, for example, a value of K.
Then, the gas to be measured is introduced into the measuring cell 1; the pulse signals, which are output from the voltage/frequency converters 5 and 6, are input to counters 7 and 8 so as to count down from the previously stored count. Finally, counters 7 and 8 are stopped when the count stored in counter 8 becomes equal to zero. The count v stored in counter 7 (see FIG. 2(a), at the stoppage of counting is used as an output datum. The count v is calculated by the equation: EQU v=Kclx,
assuming that the length of the measuring cell 1 is l, the concentration of gas is x, and the absorption coefficient is c.
According to this prior art, the influence of the fluctuations in the strength of the light source can be eliminated because a method is adopted in which the quantity of light is integrated until the definite value is reached. Thus, it is possible to accurately determine components, such as ozone and the like, having extremely low concentrations.
However, this prior art has a defect in that two voltage/frequency converters must both have an accuracy of the same order of magnitude as the above-noted concentration and, furthermore, two up/down counters must be able to count up to such a magnitude. Accordingly, such an apparatus is extremely expensive if it is necessary to make the measurement determination of components having an extremeley low concentration. For example, the determination of ozone of a concentration 1 ppm at an accuracy of 0.1% requires the determination of the quantity of light emitted from a light source at the accuracy of 10.sup.-6 and it is thereby necessary to provide a voltage/frequency converter having an accuracy on the order of 10.sup.-6. Consequently, it is necessary to use an up/down counter which can store six-digit numbers. Although an analog integrator may be substituted for the voltage/frequency converter and the up/down counter included in the circuit shown in FIG. 1, an accuracy on the order of 10.sup.31 6 is required for the integrator, so as to thereby result in an expensive instrument.